The present invention relates to radio frequency filters, and, more particularly, to filters for separating two or more predetermined frequency bands from a radio frequency signal.
The invention has been developed primarily for use in microwave communications systems designed to operate in the 500 MHz to 20 GHz frequency range, and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to use in this field.
In microwave communications systems, it is often necessary to filter a relatively broadband microwave signal into its component sub-bands. In the past, this has been done using parallel filtering systems. For example, FIG. 1 shows a filter 1 in which a single radio frequency input is split into n parallel outputs xe2x80x98Out 1xe2x80x99 to xe2x80x98Out nxe2x80x99. This is done using a plurality of resonators 2, which are coupled to each other in a known way to produce a band-limited output. The resonators can be, for example, planar resonators, cavity resonators or any suitable waveguide type resonator. At these frequencies, discrete components such as capacitors and inductors are rare, but can be used.
The difficulty with this approach is that it results in a relatively high component count. Each channel requires a complete set of resonators and couplings to filter out the entire frequency ranges that are not of interest, whilst passing the appropriate frequency range through to the corresponding output. Given the limited space in some systems, it is desirable to reduce the total number of resonators in a given filter.
In some instances, the problem is exacerbated when superconducting resonators are used. Whilst both a simultaneous reduction in space and improvement in performance over non-superconducting designs can be achieved, it is still desirable to further reduce the number of components, since each additional component places a load on the cooling engine used to keep the superconductors cooled to below their critical temperature.
In accordance with the invention, there is provided a radio frequency filter for generating at least first and second predetermined radio frequency bands from a received radio frequency signal containing signals in at least the first and second radio frequency bands, the radio frequency filter including:
a first resonant component for accepting the radio frequency signal via an input coupling;
a second resonant component operatively coupled to the first resonant component via a first coupling;
a third resonant component operatively coupled to the first resonant component via a second coupling and to the second resonant component via a third coupling;
a first output operatively coupled to the second resonant component via a fourth coupling
a second output operatively coupled to the third resonant component via a fifth coupling;
wherein the characteristics of the couplings and the resonant components are selected such that the first predetermined radio frequency band is available at the first output and the second radio frequency band is available at the second output.
Preferably one or more of the operative couplings are magnetic inductive and/or electrical capacitive couplings.
In one preferred form, one or more of the resonant components is made wholly or partly of a superconducting material.
In a particularly preferred form, one or more of the operative couplings includes one or more additional resonant components. More preferably, two or more of the additional resonant components are operatively coupled to each other.
Preferably, the filter further includes one or more additional resonant components, each being operatively coupled to one or more of the other resonant components in the filter and to an additional output, wherein additional radio frequency bands are available at the respective additional outputs.
In the preferred embodiment, the filter is configured to operate at microwave frequencies.